Support for additional fields in Tanks and filter in retrieve

[EfronC]

[POSSIBLY INCOMPLETE]

This is a small PR just to show the idea of the initial added support for extra fields in Tanks. The commit adds a new file called Constants.pm in utils, since I wanted to have a variable with the extra fields separated of the Tank code to keep the structure defined and easy to modify, and because I saw Generic.pm was full with functions, I decided to create a file exclusively for variables, otherwise both work the same.

This PR also adds support in the "Get All Tankoubons" EP to send a param name, so you can filter the result, still I'm not sure if ti should be done this way, since if someone ever gets to have so many Tanks(Which I doubt, but is possible), this might take some time to do the search, since is visiting the whole list twice at worst before paginating.

This probably has a few issues, so I keep pending for comments about changes that can be done to improve this idea.

[EfronC]

There is a bug that I just noted, where the get tankoubon EP appends an extra field "5":0 in each tank, but I've no idea why this is happening, I've tracked that it might be related to how the Tanks are created, since reducing the number of flag fields reduce the number in it, but still I've no idea where is that field being inserted.

[Difegue]

Needs additional tests imo, but this makes sense to me.
A few general code quality comments and Qs. 👍

[Difegue]

I'll have to give fetch_data a more indepth think, but did find some other stuff in the meantime.

[Difegue]

This looks markedly better to me. 👍 Just a bit more cleanup I've noticed.

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[Difegue]

Perfect, thanks!